Die casting core



March 17, 1970 R. J. PIETRYKA ETAL 3,501,320

DIE CASTING CORE Filed Nov. 20, 19s? MIXING CORE MATERIAL WITH BINDERFORMING THE CORE SHAPE CURING THE CORE PRE- FIRING THE com:

IMMERSING THE CORE IN SALT BATH I NVE NTORS United States Patent3,501,320 DIE CASTING CORE Robert J. Pietryka, Warren, and Raymond S.Amala, Oak Park, Mich., assignors to General Motors Corporation,Detroit, Mich., a corporation of Delaware Filed Nov. 20, 1967, Ser. No.684,257 Int. Cl. B28b 7/34; B22d 17/20 US. Cl. 106-3827 6 ClaimsABSTRACT OF THE DISCLOSURE A leachable core body suitable for use inhigh pressure metal casting operations and a method of preparing thesame is disclosed. The outer surface portion of a sand core isimpregnated with a water soluble inorganic sealing salt having a highmelting point such as a sodium chloride-potassium chloride mixture. Thesealing salt impregnated outer sand core layer prevents molten metalfrom penetrating the core body during high pressure metal casting. Thissealing salt mixture has a melting point higher than the temperature ofthe metal during the'metal casting operation. The core is bonded with awater soluble inorganic binder such as sodium silicate which has amelting point higher than the melting point of the sealing salt.

This invention relates to die casting cores, and more particularly todie casting cores adapted for use in high pressure metal castingoperations and including a method of forming such cores.

Conventional sand cores used in gravity cast molds are not suitable forpressure die casting of metals because the surface of such cores ispenetrated by the metal under the high pressures imposed in the process.Attempts to reduce the metal penetration by the introduction of finematerial such as silica powder into the core mix has not beensatisfactory since it increases the required binder content therebyresulting in a core having reduced solubility characteristics which isditficult to remove from the casting.

It is a primary object of this invention to provide a leachable coreadapted to withstand the pressure of the metal in high pressure metalcasting operations. It is still another object of this invention toprovide a method of forming a high strength, pressure resistant corewhich can be readily leached.

These and other objects are accomplished by a sand core having a watersoluble inorganic binder dispersed throughout and the outer surfaceportion thereof impregnated with a water soluble inorganic sealing salt.The melting point of the binder is higher than the melting point of thesealing salt. The sealing salt has a melting point higher than thetemperature of the metal in the metal casting operation. A specificexample is a sand core bonded with 4% by weight of the binder sodiumsilicate. The outer layer of the sand core is sealed to a depth of about4 inch with a sodium chloride-potassium chloride sealing salt mixture.

The core is formed by mixing the core material, such as sand, with awater soluble inorganic binder, for example, sodium silicate. Theresultant mixture is formed into a core of the desired shape by molding.The core is then cured in a microwave oven or hot cured by heating in anoven. The cured core is prefired in an oven ice to remove residualbinder water. The prefired core is then immersed in an inorganic saltbath, such as a molten sodium chloride-potassium chloride mixture. Theouter surface portion of the sand core absorbs the molten salt therebysealing the core. The core is then removed from the bath and cooled toyield a strong core body having an outer salt impregnated surface layerresistant to metal penetration in metal casting operations. Theresultant core body can be readily leached with hot water.

Other objects and advantages of this invention will be apparent from thefollowing detailed description, reference being made to the accompanyingdrawing wherein a flow diagram depicts the subject process.

The expendable cores are made with readily available foundry sands suchas the Muskegeon Lake sand and the Hemlock bank sand. The Muskegeon Lakesand contains less than 0.2% clay and has a particle size that will passthrough a 50 mesh US. Standard screen sieve. It has an American FoundrySociety designation of AFS 46-56 particle size. The Hemlock bank sandcontains 0.2 to 0.4% clay. This sand passes through a 55 mesh screen andhas an AFS rating of 4665. Although sand is the preferred core material,other ceramic powdered materials such as alumina and the like may beused.

The foundry sand is mixed with a water soluble inorganic binder. Theprimary purpose of the inorganic binder is to afford sufiicient cohesivestrength to sustain the core body intact while the core body is immersedsubsequently in a molten sealing salt bath as will be hereinafter fullydescribed. In order to hold the core body intact while immersed in themolten sealing salt bath, the binder should have, preferably, a meltingpoint higher than the temperature of the molten sealing salt bath. Theinorganic binder should have a high solubility in water so that it canbe readily removed by conventional water leaching techniques. The bindershould not dissolve appreciably in or react with the sealing salt.Sodium silicates, which have a melting point range of 1700-1900" F., areappreciably higher than the temperature of the molten salt bath and arethe preferred binders. The concentration range of the sodium silicate isfrom about 3 to 10 weight percent with the preferred concentration being3 to 5 weight percent. Cured Muskegeon Lake sand core bodies containing3 weight percent and .5 weight percent sodium silicate had a tensilestrength of 315 to 330 and 345 to 366 p.s.i. respectively. Cured Hemlockbank sand core bodies containing 3 weight percent sodium silicate had atensile strength of 255 to 285 p.s.i. This strength was sufficient tohold the core together during the subsequent immersion step in themolten sealing salt. Binder concentrations at the low end of the 3 to 10weight percent range yield cores which are more soluble whereas highbinder concentrations yield cores having higher strength. Theconcentration of the binder is selected depending upon the degree ofsolubility and the strength desired in the core body. Other suitablewater soluble inorganic binders which may be used include potassiumsilicate and lithium silicate. These alkali metal silicates have amelting point range (17901900 F.) higher than the temperature of themolten sealing salt bath, they do not react with the sealing salt andthey provide sufiicient cohesive strength to the core body.

The water soluble binders are mixed with the sand either in the drycondition followed by the addition of water, or by dissolving the binderin water prior to the addition of the sand. The binder-sand mixture isplaced in a core box and formed into cores of the desired shape byramming a rod with a pressure of about 20 p.s.i. The core may be formedby conventional blow investing techniques which are widely used.

After the core shape is formed, the core is subjected to a curing step.The core may be cured by any one of several methods. One method which ispreferred because of its speed involves exposing the core for 2 minutesin a microwave oven. Another widely used curing method is the hot cureprocedure which involves baking the core in an oven having a temperatureof about 350 F. for a period of 20 minutes. Another curing method is theconventional carbon dioxide gas curing procedure in which carbon dioxideis passed through the core body while the core is still in the core box.The curing step yields a core body having sufiicient cohesive strengthto be handled in normal operations.

After the core is cured, the core is subjected to a prefiiring step inorder to remove the residual binder water. One method of removing theresidual water is by baking the core in an oven at a temperature of fromabout 400 to 1300 F. High prefiring temperatures with this temperaturerange remove the volatiles more rapidly and more completely than the lowtemperatures within this range but have the disadvantage of causing aconsiderable reduction in core mechanical strength. The preferredtemperature range is about 400 to 500 F. for a period of about 1 hour.At a temperature of 400 to 500 F. heating 1 inch thick cores for aperiod of 1 hour adequately removes the residual water and no seriousloss of mechanical strength is encountered.

After the core has been cured and been subjected to a prefiringtreatment it is immersed in a molten inorganic sealing salt. The watersoluble sealing salt has, preferably, a melting point higher than thedie casting temperature range for the particular metal alloy which iscast. For example, the casting temperature of certain aluminum alloys isin the range of 1200 to 1250 and for this temperature range a potassiumchloride-sodium chloride mixture having a melting point of 1220 to 1280"F. is satisfactory. Salts having melting points lower than the diecasting temperature range may be used as sealants if the heat capacityand heat of fusion of the salts are great enough to prevent meltingduring the limited time of contact between the molten metal and the coresurfaces. The preferred sealing salts, however, have a melting pointhigher than the metal die casting temperature range. For the die castingof aluminum alloys having a temperature casting range of 1200 to 1250 F.a sealing salt composition formed by mixing 0 parts by weight sodiumchloride and 50 parts by weight potassium chloride is the preferredsealing salt. The sodium chloride-potassium chloride sealing saltmixture has a melting point in the range of 1220 to 1280 F. andpossesses a sufficiently low viscosity at a temperature of 1350 to 1400F. to allow the absorption of the sealing salt into the interstices ofthe outer core surface. The molten sealing salt is absorbed through thepores of the sand cores to a depth of about A; of an inch to /2 of aninch from the outer core surface during the immersion in the sealingsalt bath. It has been determined that a minimum sealing saltimpregnation depth of /s of an inch is required to effectively seal thecore from penetration by the metal during the die casting operation. Thepreferred depth is /8 of an inch to A of an inch. Cores made fromMuskegon Lake sand are successfully sealed in a sodiumchloride-potassium chloride molten salt bath by immersing the cores for2 minutes at a bath temperature of 1400 F. Cores made from Hemlock banksand which contain a small percentage of clay require a longer immersiontime, in the range of 3 /2 to 4 minutes and at 1400 F. Calcium chloride,which has a melting point of about 1422 F., may be used as the sealingsalt. However, since calcium chloride is hygroscopic, the resultant corehas a tendency to absorb water thereby requiring that cores sealed withcalcium chloride be stored under anhydrous conditions. The sealing saltsshould be soluble in water and preferably have a solubility of at least30 grams per milliliters of cold water. The following table lists agroup of salts suitable for sealing the core used in zinc die castingand a second group of salts suitable for either aluminum or zinc diecasting.

TABLEFOR ZINC DIE CASTING Solubility Melting point, (glIlS./1OO ml.

Salt F. cold water) Cadmium bromide. 1,052 57 Cadmium chloride 1, 054Calcium iodine. 1, 067 66 Calcium nitrate- 1, 042 102 Cupric bromide.928 V.S. Cupric chloride... 928 71 Ferrous chloride. 1, 237 64 Manganouschloride 1, 102 62 Potassium lead chloride. 915 Soluble Sodium iodine 1,202 159 Strontium bromide.. 1, 190 85 Strontium nitrate 1, 058 40V.S.-Very soluble.

FOR ALUMINUM O R ZINC CASTING Solubility Melting point, (gms./100 ml.

Salt F. cold water) Barium bromide 1, 557 98 Barium iodide 1, 364Calcium bromide.- 1, 410 125 Calcium chl0ride.- 1, 422 60 Magnesiumbromide l, 292 102 Magnesium chloride 1, 307 54 Manganous sulfate... 1,292 52 Potassium bromide 1, 3 54 Potassium calcium chloride... 1, 390Soluble Potassium carbonate 1, 637 112 Potassium chloride. 1, 430 35Potassium iodide. 1, 332 128 Potassium molybdate 1, 684 Potassiumsulfide 1, 544 Soluble Potassium tungstate 1, G92 52 Sodium bromide- 1,392 80 Sodium chloride.. 1, 472 36 Sodium molybdate. 1, 270 SolubleSodium sulfate 1, 622 Soluble Sodium tungstate- 1, 290 41 Strontiumchloride 1, 602 44 Mixtures of these salts may also be used.

The salt seal cores described above retain their shape and resist metalpenetration during the high pressure die casting of aluminum alloys. Thecore is removed from the aluminum casting by immersing the castings inwarm water which dissolves the core.

The following examples show different embodiments of this inventionwhich yield a Water soluble core suitable for use in high pressure metalcasting.

EXAMPLE N0. 1

Fifteen pounds of Muskegon Lake foundry sand was mixed with 68 grams ofwater for one minute. Then 340 grams of sodium silicate was mixed withthe wet sand for four minutes to form a uniform mixture containing 5weight percent sodium silicate. The sodium silicate used contained 14.7%Na O and 29.4% .SiO The sand was placed in a core box and a pressure ofabout 20 p.s.i. was applied with a ramming rod by hand to compact thecore. The core was cured (hardened) by baking in a microwave oven for aperiod of about 2 minutes. The tensile strength of the cured core was345 to 366 p.s.i. as measured by the Dietert core sand tensile tester.After curing, the core would collapse into loose sand when when immersedfor two to five minutes in warm Water having a temperature 120150 F. Thecured core Was then subjected to a prefiring treatment by heating in anoven having a temperature of about 400-500 F. for a period of about onehour. The core was immersed in a sealing salt bath having a. temperatureof 1400 F. and consisting essentially of 50 parts by weight potassiumchloride and 50 parts by Weight sodium chloride. The core was immersedin the bath for. two to three minutes during which time the sealing saltpenetrated the outer surface portion of the core to a depth of aboutinch. The core was then removed from the bath and allowed to cool. Theresultant core had a tensile strength of 50-100 p.s.i. The salt sealcore softened up sufficiently to break up after one half to one hourimmersion in warm water having a temperature of 12 150 F. The core wasused in die casting an aluminum alloy part in a high pressure diecasting operation where the temperature of the aluminum metal was 1220F. and the pressure of the die casting operation was 150 p.s.i. Duringthis metal casting operation the core remained intact and no aluminummetal penetrated the sealed core.

of, said binder having a melting point higher than said sealing salt,said sealing salt having a melting point higher than the temperature ofsaid metal in said metal casting operation.

2. A core as described in claim 1 wherein said inorganic binder issodium silicate.

3. A core as described in claim 1 wherein said sealing salt is a mixturecontaining equal weights of potassium chloride and sodium chloride.

4. A core as described in claim 1 wherein said sealer The followingtable lists Examples 1 through in is calcium chloride. which differentsand core compositions were sealed with 5. A water leachable unitarysand core for use in a molten sodium chloride-potassium chloridemixtures or high pressure aluminum metal casting operation compriswithmolten calcium chloride. ing a sand core formed from a foundry sandcomposi- Composition,wt. percent Prefiring Salt bath Cured core Sealedcore Example Sodium baked tensile Tex n Time, Temp., Immersion bakedtensile No. Sand silicate strength,p.s.1. min. Composition F. time, min.strength,p.s.1.

Muskegeonnu 3 315-330 400-500 50 NaClKCl 1, 400 2-3 50-100 Hemlock- 3255-285 400500 50 1, 400 4-5 50-100 Muskegeon 5 345-366 400-500 60 1,4002-3 50-100 "Mao 1 50-00 ..400-500 00 1, 400 2-3 5 d0 4 (320340) 1,300 30C3012 1,500 2 50-100) Example No. 2 listed in the table containing 3%tion containing 3 to 10 weight percent sodium silicate, sodium silicateand Hemlock bank foundry sand rethe outer surface portion of said corebeing impregnated quired a longer immersion time in the molten sodium toa depth of at least Ms inch with a sealing salt mixturechloride-potassium chloride salt bath to obtain the decontaining equalweights of sodium chloride and posired salt penetration in the outersurface portion of tassium chloride. the core. Examples 1, 3 and 4indicate that a concen- 6. A process for forming water leachable sandcores tration of 3-5 weight percent sodium silicate has the adapted tobe used for high pressure metal casting operpreferred tensile strengthfor the cured core whereas 1% ations comprising the steps of mixing aninorganic binder silicate yields a core with insufiicient tensilestrength. All taken from the group consisting of sodium silicate, poofthe sealed cores described above softened sufficiently tassium silicateand lithium silicate with a foundry core to break up after immersion ofone half to one hour in said to form a mixture containing 3 to 10 weightpercent warm water. of said binder, forming said mixture into a coreshape, While the invention has been described in terms of curing saidcore shape, prefiring said cured core shape at a specific examples, itis to be understood that the scope temperature between 400 F. and 1300F. for at least of the invention is not limited thereto except as de- 2hours to remove the residual binder water, and imfined in the followingclaims. mersing said prefired core in a molten sealing salt solu- Whatis claimed is: tion containing equal weights of sodium chloride andpo- 1. A water leachable unitary sand core for use in a tassium chloridefor a time suflicient for said sealing salt high pressure metal castingoperation comprising a core to be drawn into the interstices of theouter surface mahaving 3 to 10 weight percent water soluble inorganicterial by capillary action to a depth of at least its inch. binder takenfrom the group of alkali metal silicates con- 45, sisting of sodiumsilicate, potassium silicate and lithium References Cited silicatedistributed uniformly throughout, the outer Slilf- UNITED STATES PATENTSface portion of said core impregnated with a water so uble sealing salttaken from the group consisting of cad- I889O07 11/1932 Wallace 106*383mium bromide, cadmium chloride, calcium iodide, cal- 2322667 6/1943'seastone at 1O6' 38'3 XR cium nitrate, cupric bromide, cupric chloride,ferrous 3,113,360 12/1963 Nefi et 1175-2 XR hloride, manganous chloride,potassium lead chloride, 3,121,269 2/1964 Nefi 1O638-22 sodium iodide,strontium bromide, strontium nitrate, barium bromide, barium iodide,calcium bromide, cal- FOREIGN T F cium chloride, magnesium bromide,magnesium chloride, 170,677 10/1921 Great Brltallliinanganous sulfate,potassium bromide, potassium calcium chloride, potassium carbonate,potassium chloride, JULIUS F ROME Pnmary Exammer potassium iodide,potassium molybdate, potassium sul- LOREN B- HAYES, Assistant Examinertte odiu bromide sodiu hl fide, potassium tungs a s m m c o US. Cl XR.

ride, sodium molybdate, sodium sulfate, sodium tungstate, strontiumchloride and compatible mixtures there- (22 33 UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 0 0 2 Dated March 17, 1970Inventor(s) Robert J. Pietryka and Raymond S. Amala and that saidLetters Patent are It is certified that error appears in theabove-identified patent hereby corrected as shown below:

Colugn 2. 51m 59, "(179o-19oo 1a)" should read (1790 -1900 +F.) column4, in Table For zinc Die Casting "iodine", each occurrence, should readiodide Column 5, line 7, "150 p.s.i." should read 1500 p.a.i. --1 line32, after "1%" insert sodium SIGNED AND SEALED JUL 281970 Anest:

Edward M. Fletcher, 11%

11mm Gemissioner or Pat n

